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Anodes Soles MP0 Generator INVENTOR Ernest C. Okress ATTORNEY UnitedStates Patent 3,355,605 CROSSED FIELD PLASMA DEVICE Ernest C. Okress,Elizabeth, N.J., assignor to American Radiator & Standard SanitaryCorporation, New York, N.Y., a corporation of Delaware Filed Sept. 23,1963, Ser. No. 310,608 18 Claims. (Cl. 310-11) This invention relatesgenerally to (thermal to electric) gaseous energy converters and moreparticularly to a cross field non-equilibrium plasma-type referred togenerally as a non-equilibrium magneto-hydrodynamic (MHD) generator.

Common MHD generators utilize thermodynamic equilibrium or thermalionization of the Working medium, which is commonly seeded (combustion)gas in open cycle operation. Because of the extremely high temperatureof the working medium which would be required for MHD generatorconditions with gas alone (e.g., 5800 F. 1 mho/m., 6900 F. mho/m. and8500 F. 100 mho/m.) to ionize most gases because of the relatively highionization potential required to achieve the desired electrical gasconductivity, seeding material (e.g., 1% molar alkali, such as potassiumor cesium), which ionizes at a relatively lower gas temperature, isgenerally added to the gas. Unfortunately, even with seeded combustiongases the temperature is still too high for continuous full (100%) dutyand long life operation at MHD conditions (e.g., 3500 F. l mho/m., 4200F. l0 mho/m., 5100" F. 100 rnho/m.) when compared with non-equilibriumMHD conditions (e.g., for appropriately seeded nonatomic gas at 2200" F.an order of magnitude higher electrical conductivity can be achievedcompared with combustion seeded gas at 5000 F.).

The purpose of ionization of the working medium is to obtainsuificiently high electrical conductivity for the purpose of achievingoptimum MHD generator performance and tolerable MHD generatorsizeespecially with respect to its dominant physical parameter, theelectrical conductor or ionized gas duct length. Aside from the problemof recouping of the seeding material, particularly in open cycleoperation, crucial material limitations arise because of the high gastemperatures associated with combustion MHD generators which tax theendurance of available (electrode and confining wall) materials to sodrastically limit the duty of operation and life of the MHD generatorand associated system as to require impractically low values of theseparameters. Commercial apparatus is invariably continuous or full (i.e.,100%) duty, relatively high impedance, alternating current type, withgenerally long reliable life characteristics. Hence, power supplies tobe competitive with state of art, must not only approach thesecharacteristics, but must do so reasonably economically in all respects.Consequently, non-equilibrium gas ionization resolves the major problemsof MHD generator and associated components of higher electrical gasconductivity and lower gas temperature than is possible with equilibriumor thermal means. Therefore, it offers much better prospects forrealizing most of the power source requirements than thermal MHDgenerators do, in spite of the formers comparative primitive state ofdevelopment.

As an alternative, therefore, to thermal (i.e., equilibrium) means ofgas ionization, a non-equilibrium and in particular novel electricalmeans are utilized in this invention. Equilibrium vs. non-equilibriumgas ionization denotes whether the ion species temperature is comparablewith or higher than, respectively, the gas or more properly the relatedstagnation temperature (by virtue of the isentropic expansion relation).

It is an object of this invention to provide a device which can operateat continuous or full (i.e., 100%) duty and with long life not limitedby thermal endurances of available materials.

It is another object of this invention to provide a device which can becontinuously or intermittently operated at arbitrary pulse duration andaverage power, up to and including duty.

It is another object of the invention to provide a device which cangenerate either direct current or alternating (i.e., sinusoidal) currentcomponent at commercial or domestic frequencies and voltages.

It is another object of the invention to provide a device whose directcurrent or alternating current output voltage is suitable for operationwith commercial distribution systems or transformers, respectively.

It is another object of this invention to provide a device which doesnot require (condensable) seeding of the gas, but which can be useddepending upon the thermodynamic cycle required for a specificapplication (e.g., Rankine vs. Joule cycle).

It is another object of this invention to ionize the gas by crossedfield electron beam injection, at the expense of a tolerable or smallfraction of the power output of the MHD generator.

It is another object of the invention to provide crossed field electronbeam injection along the interaction duct, especially in sufficientlyhigh magnetic field regions thereof in a convenient and practicalmanner.

It is another object of the invention to minimize excess ionizationespecially at electron injection ports by use of controlled cross fieldelectron guns and so minimize recombination energy losses.

It is another object of this invention to supplement the degree ofionization atforded by the crossed field electron guns by periodicelectron heating on account of the inherently required pulsed ionizingelectric field in the guns sufficiently below the sparking limit toavoid excess ionization. This feature requires extremely short (i.e.,order of state of art millimicrosecond) pulse durations, but atcorresponding high duty to mitigate the correspondingly rapidrecombination rate and so as not to depart significantly from continuous(full duty) operation conditions.

It is another object of this invention to provide a device which isefficient and competitively simple in design, fabrication and operation.

It is another object of this invention to provide a device andassociated closed cycle system which is portable in nature.

It is another object of this invention to provide a closed cycle systemwhich has no moving mechanical parts.

It is another object of this invention to provide a device wherein thedegree and uniformity of ionization of the gas is under control andindependent of the temperature of the gas.

It is another object of this invention to provide a device wherein thegas is ionized in a uniform and continuous manner.

It is another object of this invention to provide a device wherein anMHD generator and an MHD motor (or pump or accelerator) areself-exciting.

It is another object of this invention to provide a device which canstart operating practically instantaneously.

It is another object of this invention to provide a device that is notaffected by its orientation.

It is another object of this invention to provide a device that isoperable on or in the land, sea, air or space.

It is another object of this invention to utilize an electrodeconfiguration best suited to but not rigidly restricted to high Hallparameter operation.

It is another object of this invention to provide a system which can bemade to operate open cycle at supersonic or hypersonic gas velocity asan efficient electrical propulsion device for space applications.

It is also an object of this invention to provide a type of cathode suchthat its non-thermal and thermionic electron yields are adequate andwhereby the thermionic portion is derived from cathode heating by thehot flowing gas in the duct.

It is also an object of this invention to achieve the required gasionization by simultaneous electron beam and periodic ionizing electricimpulses and thereby mitigate the dependence on a finite electronsupply, tolerance of recombination coefiicient and electron mobilitywhich are so inherent in the former means of gas ionization with respectto the latter.

It is also an object of this invention to utilize a Hall (segmented)electrode rather than a Faraday (segmented) electrode configuration dueto desire for operation at high Hall elfect parameter or high magneticfield, because the Hall configuration requires less control, is morereliable and simpler than the Faraday configuration. Furthermore, theHall configuration is not subject to the requirement of fixed loading ormultiple loads. Finally, the Hall configuration provides a significantreduction in electron retardation or larger electron braking on accountof employment of the magnetic field with transverse electric field. Alsoa high magnetic field, and relatively low gas velocity contemplated,improves the momentum transfer between electrons and gas atoms.

It is another object of this invention to provide gas ionization meanswhich is not an ionizing radiation hazard.

It is another object of this invention to provide a device which canoperate at minimum gas temperature consistant with desired thermodynamic(cycle) efficiency and material endurances for full duty long lifeoperation.

It is another object of this invention to provide a MHD motor or pump orcompressor or accelerator so as to eliminate mechanical moving parts inthe closed system.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the apparatus becomes better understoodby reference to the following detailed description when considered inconnection with the accompanying drawings, wherein:

FIGURES 1A, 1B, 1C and 1D are schematic and vector diagrams of variouselectrode configurations appropriate for use in MHD generators;

FIGURE 2 is a flow diagram of a closed cycle MHD generator;

FIGURE 3 shows salient MHD quantities involved in the description of MHDenergy converter operation;

FIGURE 4 is a schematic representation of an MHD generator or an MHDmotor in accordance with the principles of the invention;

FIGURES 4A through 4E illustrate alternate electrode pair means ofapplying pulsed ionizing voltage to interaction space of the MHDgenerator or MHD motor illustrated in FIGURE 4;

FIGURE 5 is a perspective view representing schematically a mechanicalnozzle or convergent-divergent gas duct with electrodes for an MHDgenerator of the type shown in FIGURE 1C and FIGURE 4;

FIGURE 6 is a longitudinal sectional view of an illustrative embodimentof an MHD generator in the form of a straight constant cross-section gasduct;

FIGURE 7 is an external elevational view of the device shown in FIGURE6;

FIGURE 8 is an external end view of the device as viewed in FIGURE 6;

FIGURE 9 is a partial view of an electrical connector of low heattransmission characteristics as viewed along the line 99 in FIG. 6;

FIG. 10 is an clevational view of an insulating (e.g.,

circular) spacer taken as indicated by arrows 1tl-10 in FIG. 6;

FIG. 11 is a cross-sectional view taken along the line 1111 in FIG. 6;

FIGS. 12 and 13 are fragmentary views of duct walls grooved andpreshaped to receive short-circuiting straps as shown in FIGS. 6 and 11;

FIG. 14 is a schematic showing of an arrangement for applying a magneticfield to a gas duct by means of ordinary (or superconducting) solenoidelectromagnet in which wrought iron pole pieces are saturated and,consequently, no yoke is used;

FIG. 15 is a cross-sectional view taken along the line 15-45 of FIG. 16;

FIG. 16 is a schematic cross-sectional representation of a compoundcrossed field electron gun with strip cathodes and multiple vane controlelectrodes for injection and modulation (for alternating currentoperation of the MHD generator) of an electron beam in a gas duct;

FIG. 16a is a cross-sectional representation of a crossed field electrongun for direct current operation of the MHD generator, this structurebeing substituted for the electron gun of FIG. 16 when direct currentOperation is desired rather than alternating current operation;

FIGURE 17 is a schematic plan view taken along the line 17-17 in FIGURE16;

FIGURE 18 is a schematic cross-sectional representation of a compoundcrossed field electron gun with multiple plasma-cathodes for injectingan electron beam into a gas duct;

FIGURE 19 is a schematic plan view taken along the line 1919 in FIGURE18;

FIGURE 20 is a schematic cross-sectional representation of a dualcompound electrostatic electron gun with strip cathodes, for injectingelectron beams into a gas duct along the direction of the appliedmagnetic field in an MHD generator (or motor);

FIGURE 21 is a schematic cross-sectional representation of a dualcompound electron gun with plasma. cathodes, for injecting electronbeams into a gas duct along the direction of the applied magnetic fieldin an MHD generator (or motor);

FIGURE 22 is a schematic representation of a helically wound gas duct ofan MHD generator (or motor) with ordinary (or superconductor) solenoidalwindings for providing the required magnetic field, an outer solenoidbeing partially cut away. Such a structure is more complex than itslinear counterpart for sake of compactness;

FIGURE 23 is a schematic representation of the device as shown in FIGURE22 in longitudinal cross-section;

FIGURE 24 is a schematic cross-sectional representation of a helicallywound gas duct of an MHD generator (or motor) with saturated magnet ironpole pieces and ordinary (or superconducting) solenoids disposed betweenadjacent turns of the helically wound duct. Such a structure is morecomplex than its linear counterpart for sake of compactness;

FIGURE 25 is a schematic diagram of typical electrical connectionssuitable for connecting an MHD generator of the type shown in FIGURE 1Cto an MHD motor of the type shown in FIGURE 1D.

Similiar reference characters refer to similar parts throughout theseveral views of the drawings.

Briefly, in this invention thermal energy in a suitable unseeded gasderived from a primary heat source (e.g., fossil fuel, solar energy orgas cooled nuclear reactor) can be directly converted into commercial,direct or alternating current electricity, depending upon the ultimateapplication. Crossed field electron beam and interdependent periodicionizing electric field impulse gas ionization means are utilized toprovide a device whose gas temperature does not tax the endurance of theassociated materials. Thus an MHD device which can operate continuouslyat full (i.e., 100%) duty and still have a long operating life isprovided.

In this invention a suitable gas is nonequilibriumly or electricallyionized to make it optimumly electrically conducting, by crossed fieldmeans, involving electron injection, and/or by periodic ionizingelectric field impulse means, thereby enabling the device to convertdirectly a major part of the thermal or kinetic energy, of the gasimparted to it by a suitable primary heat source, directly intoelectricity.

The rate of extraction of enthalpy from the gas by the MHD generatorderives from the usual energy balance, which shows that it dependsstrongly (i.e., square) on the gas velocity-magnetic field product, fora given efi'lciency, conductivity, Hall parameter, degree ofnonuniformity and ion slip, neglecting heat loss to the exterior andchanges in kinetic energy in the gas.

The energy transfer means is in accordance with the equivalent Hall orFaraday generator principle, taking cognizance of the fact that whereasthe energy transfer via a metal electrical conductor is by virtue of thepositive ions anchored to the crystal lattice, in a gas electricalconductor energy transfer occurs only by virtue of attractive electricalforces and predominately axially directed collisions between(unanchored) ions and (neutral and excited) gas molecules.

This invention is also capable of directly converting electricity tomechanical (directed kinetic) ener y of the gas, in accord with theequivalent Faradays motor principle. The resulting MHD motor or pump,accelerator or compressor does not depend on any moving mechanical partsand, therefore, it eliminates the need for any mechanical components inthe whole system. The energy required to operate the MHD motor isderived from a tolerable fraction of the MHD generator output.

Either fossil fuel, direct solar energy or a gas cooled nuclear reactormay serve as the primary heat source for heating the gas or workingfluid of the proposed energy converter. The electrical power output canbe con tinuous or full (100%) duty (i.e., direct current), orintermittent (i.e., pulsed), or alternating (e.g., sinusoidal) cur rent.In those instances where alte'nating current or modulation is desired,low drive of a voltage controlled electrode system of the crossed field(or electrostatic) electron gun (used for injecting the electrons intothe gas at high efficiency for complete absorption and ionization of thegas) is utilized. In the case of pulsed modulation, various pulsedurations and repetition rates can be utilized. In the case ofalternating current modulation, various frequencies, includingcommercial or utility frequencies, can be realized.

The life of the non-equilibrium MHD motor (i.e., pump or compressor oraccelerator) is not limited in duty or life by the state-of-artmaterials of the (metallic) electrodes and confining (dielectric) ductwalls. This is a consequence of the fact that highly efiicientnonequilibrium electrical gas ionization (i.e., 90%) is utilized,instead of equilibrium or thermal (high temperature) gas ionization andthe required doping or seething with easily ioniza-ble material (e.g.,potassium, cesium, and the like). However, even with seeding, gastemperature of 3500 to at least 4000" F. are required by thermal orequilibrium MHD generators in obtaining desirable electrical conductivity of the seeded gas. In this invention the temperature of thegas is approximately 2500 F. as it need mainly be adequate for goodthermodynamic (cycle) efficiency (e.g., -45%) and, suitable with respectto duty and life of the primary source of heat, such as a gas coolednuclear reactor. It is possible to obtain high generator efficiency(i.e., theoretically about 95%) and high cycle efficiency (i.e., '-45%at 2500 F.).

The upper practical limit of the generator efiiciency is dependent uponthe maintenance of an adequate uniformity and stability, -(e.g.,avoidance of spark channels) in the plasma. Spark channels andconsequently excess channel ionization can be alleviated, especially inthe crossed field guns, :by resort to periodic (rectangular) impulses ofextremely short (i.e., order of a nanosecond) pulse duration and risetime (within state of art capability of a decinanosecond andcentinanosecond, respectively) at high duty, so as to mitigate theeffects of recombination rate and provide virtually continuous (fullduty) operation. In fact, pulse duration somewhat longer than this(e.g., order of a centimicrosecond) serves to substantially supplementthe degree of gas ionization afforded by the crossed field electroninjection since all the electrons in the gas are involved.

Due to the use of electrical non-equilibrium ionization means, the MHDgenerator and MHD motor are capable of self excitation and possess rapidstarting characteristics. The performance features of the MHD generator,particularly power density and conversion efficiency, are best realizedespecially with respect to the electron beam gas ionization means alonewhen the Hall (effect) parameter, B (i.e., product of electron mobility,and magnetic field B), is dominant. The requirement of high electronmobility, including considerations relative to the primary heat source,as well as to (high) specific heat/thermal conductivity and (low)molecular weight, on account of thermodynamic and aerodynamiccondsiderations, are satisfied by using noble gases at low pressure.High magnetic field reduces generator size, reduces losses proportionalto Wall area and enables quicker extraction of power. There is also asignificant reduction in electron retardation (i.e., much largerelectron braking) on account of the magnetic field employed with atransverse electric field. Furthermore, a high mag netic field andrelatively low gas velocity improves the momentum transfer betweenelectrons and gas atoms.

Need for high magnetic field on account of need for high Hall effectparameter calls for a Hall configuration Which requires less control, ismore reliable and simpler than the Faraday configuration and is notsubject to the complexity of multiple loads or fixed loading (constant Ktype). Low gas pressure contributes toward minimizing the predominantrecombination-attachment losses. It also reduces wall heat loss andviscous pressure loss. On the other hand, heat exchange size andnon-equilibrium ionization time are increased.

In this invention, electrical non-equilibrium ionization of the gas isefiiciently initiated and maintained with negligible, if any, gas flowobstruction or at the expense of magnetomotive force of the magnets.This gas ionization means is achieved dually by crossed field electronbeam injection means supplemented by periodic ionizing electric impulsemeans in the guns and interaction duct. While the former ischaracterized by a finite number of electrons the latter involves allthe electrons in the gas.

The electron beams can be injected into and guided in the MHD generator(or MHD motor) interaction space, for example, by means of an applied(electric-magnetic) crossed field (short, long, space charge, ramp ormagnetron optic) electron guns, it being understood however, that theelectron emitting source is not restricted to a solid state type (e.g.,plasma). The term short or long" crossed field electron gun refers tothe length of the electron guns cathode along the electron orbitdirection in vacuum. When the vacuum length is less than a half (and inparticular a quarter) cycle the gun is termed short optic. Whenotherwise (i.e., several half cycles) the gun is termed long oradiabatic optic. The electron beam can also be injectedelectrostatically into the MHD generator (or MHD motor) interactionspace, along the (applied magnetic field axis which acts as a beamfocusing field). However, this is at the expense of considerable design,fabrication and operational complications and disadvantages andadditional magnetomotive force requirement on the part of the magnet. Inany case,

the electron beam must permeate the gas interaction space completely andcontinuously, due to the inherently rapid gas ionization decay rate andfor sake of uniformity, especially at injection sites. This meansprovides a finite number of electrons into the gas and so its ability toprovide the desired gas electrical conductivity is correspondinglylimited.

In contrast, the applied periodic rectangular ionizing electric impulsesare applied to all the electrons in the gas since this field isimpressed between the electron emitters (i.e., cathode) and electroncollectors (i.e., anodes or accelerators) and are of such duration(e.g., order of a centimicrosecond) that, together with the injectedelectron beams, competitive non-equilibrium and uniform gas electricalconductivity (e.g., 10 to 1000 mho/m.) may be achieved without sparkingand consequeut excess ionization. This supplementary means is inherentin the operation of the system. It involves essentially impartingperiodically a relative low average energy to all the electrons in thegas for such brief intervals (e.g., order of centimicroseconds fortypical MHD conditions) so as to suificiently ionize the gas, but notexcessively so as to cause sparking or arcing and energy dissipation.The net energy expended in producing each ion pair decreases as theelectron temperature becomes much greater than that of the gas.

This invention avoids all of the thermal or equilibrium MHD generatorsmaterial problems by using a gas temperature that is substantially lowerand below the thermal endurance limitations of materials, consistentwith reasonable thermodynamic (cycle) efficiency. With this invention itis not necessary to seed or dope the gas and, naturally, the problem ofrecouping the seeding material is eliminated. However, it is not hereimplied that seeding or doping the gas cannot be used, since there maynot be need for recouping the seed material in closed cycle operation.

Although the proposed system can be used with various (fossil fuel, gascooled nuclear reactor or solar) primary heat sources due to therelatively low gas temperature, it is very adaptable for use with anadvanced gas cooled nuclear reactor having unclad fuel elements asprimary heat source, taking cognizance of the fact that state of artnuclear reactors operate in the vicinity of 1,0- F. Thus, it can bereadily seen that it is Well suited for remote applications and even forvehicles for land, sea, air or space. In the latter application aRankine rather than a Joule cycle is indicated, hence a condensibleworking medium (e.g., Ce) may be incorporated.

As mentioned previously, this invention also embraces an MHD motor orpump or accelerator or compressor having no mechanical moving parts andwhich is capable of moving gas at velocities from subsonic, tosupersonic and even hypersonic magnitudes. While the MHD motor, pump,accelerator or compressor component is used for moving the gas in aclosed cycle system for generation of electric power, it can be adaptedfor use in an open cycle system for providing MHD propulsion of spacevehicles.

The normally linear structure of the MHD generator (or MHD motor), forelectrical gas conductivities significantly less than about mho/m., canbe rolled into a helical structure as a conventional helix delaystructure of an appropriate electronic tube (e.g., traveling wave tube)or folded into a meander structure, as an interdigital delay line (ofsuch an electronic tube) in order to avoid unwieldly structures andobjectionable and even impractical overall dimensions, while utilizingsuch length of the gas duct, or electrical conductivity of the gas. Insuch compact forms, conservation of magnetomotive forces can also beaffected in the production of the required uniform magnetic field in theinteraction space. The helical structures can be referred to as periodicmagnet helix MHD generators (or motors). While the helix (or meanderalternative) duct structures are not here considered further, it shouldbe remembered that this invention can also assume these configurations.

In the type of MHD generator configuration disclosed herein (i.e., Hallgenerator, for the sake of one of the two means of gas ionizationcrossedfield electron beam injection) the voltage and current or power is takenoff longitudinally or axially by appropriate electrodes in theinteraction duct, which conveys the electrically conducting gas. Thetransverse currents provide the necessary physical resistance to the gasflow. In effect, the physical power density expended by the gas flow bypushing against the electromagnet forces is represented by the sealerproduct v BI (where v denotes the gas velocity, B denotes the magneticfield and I denotes the transverse current density). Substantiallysimilar structural configurations can be used in the MHD generator andthe MHD motor or pump or accelerator or compressor, except in the lattercase appropriate structural and electrical alterations are dictated andthe voltage is applied across the electrodes of the desiredconfiguration, illustrated in FIGS. 1A, 1B, 1C, 1D.

In the MHD generator, the electrically conducting gas is heated to theappropriate temperature by the primary heat source and forced throughthe magnetic field to generate electric power output in the mannerdiscussed in the text and in accordance with the Hall or Faradaygenerator principle in which the major current flows perpendicular tothe strong magnetic field. Furthermore, it is essential that the energyloss to the duct walls be kept to a tolerable low level compared withthe generated or dissipated energy in the volume of the gas. Thisimplies maximizing the gas pressure-cross section product. Finally, itis essential to maintain uniform ionization at an adequate level. In thecase of the MHD motor, the Hall or constant K electrode configurationalso can be used with appropriate external voltages impressed across theelectrodes. It should be mentioned that in the Hall configuration, withrespect to motor or generator, non-uniformity has greater influence inreducing the effective gas conduc tivity and useful range of the Hallparameter. In any event, the electric field so established works withthe impressed magnetic field to move the electrically conducting gasalong through the duct by cross field interaction at the desiredvelocity.

The output ends of the MHD generator and MHD motor are combined in aunitary structure to form a regenerative heat exchanger (or regeneratorand preheater, respectively), wherein heat from the residual hot gasdischarged from the MHD generator is conducted to the colder gas leavingthe MHD motor or pump or accelerator or compressor to preheat the gaswhich is on the way to be reheated by the primary heat source.

It is appropriate to clarify the mechanism involved in crossed fieldelectrically conducting gas interaction in the following manner: Sinceonly electric charges can interact with electric and magnetic fields, anefficient mechanism must exist for transferring the directed kineticenergy from the charged particles to the neutral gas molecules and viceversa. Otherwise, the efliciency of the MHD generator or MHD motorcannot exceed the fractional ionization. The reason for this lies in thefact that most of the kinetic energy in the gas resides in the flowingneutral gas molecules.

Energy transfer in this case can result only from attractive forces andcollisions between molecules in the gas conductor. Charges are generallyequally divided in number between electrons and ions, therefore, intransferring their energy to an external load they are decelerated atthe expense of their directed kinetic energy. However, for the sake ofhigh conversion efiiciency, the essentially neutral gas molecules mustalso be decelerated by transferring energy to the charges. Now, theelectron mobility is high relative to that of the ions and hence theelectrons are responsible for the major electric current flow. Also,electrons under the influence of an appropriate electric fielddecelerate promptly, due to their low mass, and form a space charge.This electron space charge exerts electrostatic forces on the positiveions. The latter, because of their relatively enormous mass, compared tothat of the electrons, are correspondingly less influenced by theapplied electric field. Thus, a positive ion space charge also forms.The electron and positive ion space charges constitute the plasma. Now,the collision cross section of positive ions and neutral gas moleculesare similar. Hence, the positive ions by virtue of efiicient axiallydirected momentum, transfer via direct (axially directed) elasticcollisions appropriate forces on the neutral gas molecules. Hence,reliance on the electrons alone for transferring energy to neutral gasmolecules (being a relatively inefficient mechanism on account of theelectrons relatively inferior collision cross section) is dispensedwith. Thus, the gas stream, in forcing the charges through the appliedfields, is decelerated and, furthermore, continually replenishes thesecharges. The reduced kinetic energy of flow appears as electric energyat the terminals of the generator in accord with Faradays generator principle. Furthermore, the effective collision cross section of ions withelectrons is enormously greater (e.g., by thousands of times) than thatbetween electrons and neutral gas molecules. Consequently, the electricgas conductivity at about a tenth of one percent ionization may beconsidered to be effectively equivalent to that at complete ionization.Therefore, essentially three mechanisms of gas retardation or braking bycharges may be considered to be as follows: Namely by electrons alone,by positive ions alone and by electrons and positive ions together, withthe gas atoms.

The relative collision cross section of electrons and positive ions withgas atoms are such that Whereas an enormous (typically number ofcollisions with gas atoms are required in the case of electrons toaffect significant momentum transfer, only a few collisions are requiredin the case of positive ions. Hence electrons alone affect only amarginal braking of the gas even in the case where the magnetic field ishigh and gas velocity low.

Electrons experience strong interaction or coupling with the magneticfield, and so are promptly braked by it, due to their relatively smallmass. In contrast, positive ions experience no relatively significantinteraction or braking by the magnetic field because of the stronginteraction or coupling between the transverse moving positive ions andthe gas atoms by virtue of their comparable masses and collision crosssections. Nevertheless, because of their comparable collision crosssections, positive ions experience strong interaction or coupling withthe gas atoms and hence effectively retard the gas atoms by directedelastic collisions. Strong coupling or interaction between electrons andpositive ions by virtue of their electrostatic forces serves to jointhese two mechanisms so as to effectively retard the gas atoms. In anyevent, little momentum transfer between charges and gas atoms canaccount for the prevailing MHD power generated since it is such a smallfraction 1%) of the total energy associated with the gas flow. Withtransverse electric field, a significant retardation of electronsoccurs. This dictates shorted electrode pair configuration and henceHall rather than Faraday type generator. The foregoing may also beviewed with respect to the degree of the Hall current involved. In thecase of no Hall current, every time a Hall field develops which couplesthe positive ions and electrons together, the electrons are responsiblefor the electric current to the extent of the cathode capacity. Themomentum transfer is predominantly through the positive ions and gasatom collisions. In contrast, with Hall current, the positive ions andelectrons act independently. The electrons are still responsible for theelectric current. The momentum transfer is, however, predominantlythrough electron and gas atom collisions.

To provide the required optimum electrical conductivity of the gas at amoderate gas temperature (e.g.,

l0 mho/m. and more at about 2,500 F. for the sake of good thermodynamicperformance and generator length) so as not to tax the endurances of therefractory materials, departure from the prevailing thermal ionizationpractices in several respects is necessary. For the sake of tolerablerecombinations/attachment loss the average ion density should be madelow. However for the sake of power output density, the electron mobilityshould be made high. These requirements suggest a low gas pressure andessentially (pure) monatomic noble gas, possessing low molecular weightand high specific heat/thermal conductivity. Polyatomic moleculesdegrade electron energy by inelastic collisions. To ionize the gas,Without experiencing a serious frictional pressure drop andmagnetomotive force sacrifices, the (eflicient) process of controlledcrossed field electron beam injection into the gas is proposed as one oftwo interdependent gas ionization provisions as the gas enters the crossfield interaction space of the MHD generator (or MHD motor). Aspreviously described, this gas ionization means is supplemented byapplication of periodic ionizing electric field pulses below thesparking limit (i.e., order of decimicrosecond) to ionize the gas to therequired degree in order to achieve the desired electrical gasconductivity (i.e., 10 mho/m.). The gas ionization so produced can bemade relatively uniform across and along the interaction region to avoidexcess ionization (e.g., at electron injection orifices, or by too longionizing voltage pulse, depending on ionization means) andsimultaneously combat the relatively rapid ionization decay rate. Thetime for ionization to reach the desired level is extremely short(augenblicklich), so that for typical MHD generator operation excessionization and sparking would be expected in the order of a microsecondand inadequate ionization in the order of a nanosecond. Also, the decayof the ionization from its established level is likewise rapid, so thatthe electrical gas conductivity under typical MHD generator operationcan be reduced to such an extent that it is no longer useful in theorder of a demisecond. Control of the gas conductivity by the proposedmeans are such that it can be maintained within an optimum range. Thatis, not too low, for good efficiency and tolerable generator size(especially duct length) at a given power, and also not too high, so asto avoid severely distorting the cross sectional flow front of the gasstream and severe entropy changes due to excessive Joule heating lossesfrom currents in undesirable paths. Using the crossed field electronguns and periodic ionizing electric field pulses here disclosed, it ispossible to achieve uniform ionization and electron injectionefficiences approaching high values (e.g., percent).

For the sake of high power density, a high magnetic field is indicated.This measure also reduces the MHD generator (and motor) size, includinglosses proportional to wall area. Of course, a high magnetic field mustbe bought at the expense of magnetomotive force across the gas gap,which is a serious problem even for the electrical gas conductivitycontemplated. As a matter of fact, the minimum gas electricalconductivity is largely governed by the magnet solenoid regardless ofits resistivity, due to cost and weight factors. In any event, both highelectron mobility and high magnetic field means a high Hall (effect)parameter. Hence, longitudinal or axial voltage and current or powerextraction, without restriction on loading factor, is indicated. TheHall (or H) configuration illustrated in FIG. 10 seems best suited tomeet the foregoing requirements. Hence, this indicates the use ofsegmented electrodes with desirable single high impedance load orvoltage operation, suitable for commercial applications. It should bementioned that non-uniformity is a more significant factor in this typeof generator than displayed by the other types mentioned. Thisrestriction tends to reduce the useful range of the Hall parameter andeffective gas electrical conductivity. Furthermore, for high efficiencya high electron mobility and high uniformity are required. Hence, idealperformance may be more difficult to achieve as the Hall parameter isincreased. Also, at high Hall parameters the ions begin to absorbsignificant energy and begin to display significant ion mobility or ionslip. In other words, conditions in which the mean free path of theneutral gas molecules becomes large enough so that they begin to passthrough the plasma without appreciable interchange of momentum.

Although the gas velocity should also be high, for the sake of highpower density and (minimum) generator size (especially duct length),there is the frictional pressure drop and other aerodynamic problems tobe considered. As a preliminary compromise, therefore, to reduce thisdrop to a tolerable level, a gas velocity in the subsonic range isindicated; it being understood, however, that other than subsonic gasvelocities can be used.

In this invention, the generator efficiency (i.e., theoretically 90%)and thermodynamic or cycle efiiciency (i.e., order of 50%) of energyconversion are correspondingly high. Furthermore, the device is capableof continuous (i.e., 100%) duty operation with long competitive (powerstation) life, since the materials of electrodes and confining walls arenot taxed to or beyond their thermal endurance by the relativelymoderate (i.e., order of 2,500 F.) operating gas temperature.

By means of low drive (power) modulation of the control electrode of thecrossed field electron gun and periodic ionizing electric field,intermittent (i.e., pulsed) or alternating (sinusoidal) currentcomponent power output can be obtained. Continuous (i.e., 100%) dutydirect current output is available in the absence of such modulation. Aspreviously mentioned, crossed field gun operation involves pulsing theionizing electric field at such short pulse duration (e.g., order ofdecimicrosecond for typical MHD generator operation) that sparking andnonuniformity in gas ionization are not encountered and at such highduty as to mitigate the deleterious ionization decay rate so as toapproach full duty operation of the MHD generator.

The required high purity of the noble gas is maintained by effective(physical, chemical and/or electrical) continuous gettering of gasimpurities (e.g., polyatomic molecules) in the interaction space of theMHD generator or MHD motor or pump or accelerator or compressor so asnot to significantly degrade the electron energy by inelasticcollisions. The methods of gettering can include electrical crossedfield methods as well as purely physical and chemical methods.Continuous monitoring or measuring of the actual purity of the gas isnecessary and can be accomplished by means of magnetron and massspectrometer, together with appropriate diagnostic determinations ofelectrical conductivity and other relevant parameters of the plasmaassociated with the performance of the devices disclosed.

With reference to the FIGS. lA-lD, there is shown schematically variouselectrode configurations and associated vector diagrams which can beemployed in MHD energy converters. FIG. 1A will be termed the elementaryFaraday (E) type configuration. This is a transverse or shunt type, thatis, the voltage, current or power extraction in the case of thegenerator of this type is transverse. The electrodes are continuouslyconductive along the length of the gas duct and are connected to asingle load. The (not necessarily) uniform magnetic field intensity, inthe MHD interaction space is indicated perpendicular to the plane of thepaper and the gas velocity, v is represented by an arrow as directed.This configuration of magnetic field intensity and gas velocity isassumed to be the same in each of the FIGS. lA-lD.

FIG. 1B will be termed the multi-load-shunt (or 1r) Faraday segmentedtype utilizing transverse or shunt power extraction. The electrodes aresegmented and each electrode pair is connected to a separate isolatedload R FIG. 1C will be termed the Hall (or H) type. It utilizeslongitudinal (or series) power extraction. The electrodes are alsosegmented and connected to a single load, while the individual opposingelectrode pairs are strapped as indicated.

FIG. ID will be termed the modified Faraday segmented or constant K (orK) type. It also utilizes longitudinal (or series) power extraction. Theelectrodes are also segmented and connected to a single load in a wavepattern, a cathode of one opposing electrode pair connected to the anodeof an adjacent pair, as indicated. The electric field in the interactionspace in this type is in a fixed direction, independent of the loadingfactor, K. The value of the loading factor is restricted to that valuewhich makes the strapped electrodes of equal voltage before strapping.

It can be shown that the performance of each type of MHD converter withrespect to power density and efiiciency depends upon the Hall parameterboth for electrons and for ion-electrons. The Hall parameter for ions isdenoted by B, where ,u is the ion mobility of the positive ions of thegas and B denotes the magnetic field intensity. The Hall parameter forelectrons is denoted by E, where M is the electron mobility as referredto previously. In the case of (n B)( t B) l, the performance of alltypes is poor. This is because the collision mechanism between positiveions and neutrals is weak.

In the case of (,t B) (,u B) 1, the performances of the four types areindividually differentiated, depending upon the value of B. It should benoted that when (#e (MB) the E-type has good efiiciency for low valuesof rt the ir-type has good efficiency for both high and low a but not asgood as the K type at low values of K. In addition, the performance ofthe 1r-type is independent of the value of B. The efficiency of the Halltype under the condition of .t B)(,u B) l, is good for high values ofTherefore, it must operate at peak efficiency. However, it has twice theduct length of the rr-type for large B and the same efficiency. Theefiiciency of the K-type is better than that of the 1r-type for highloading (K 0.5). Its power density under load is the same as that of thevr-type. However, as noted above, this type is restricted to a specificvalue of K. For a particular value of K, the 1r-type and the K-type workwith the same power density and related parameters.

The E-type performs well with respect to etficiency and power density atvalues of B 1, that is, at moderate or small (i.e., ,u B 0) values ofHall parameter for electrons. The 1r-type performs well not only atintermediate (i.e., Kn B l0) Hall parameters, but in fact at allpositive values of ,u B (i.e., Oe B- a) and so is less restricted thanE-type in this respect. The Hall-type performs best when the Hallparameter for the electrons is dominant, that is, when 10 n B 10. All ofthese three types have high efficiency and high power density in theirgood performance range.

Closed cycle system FIG. 2 shows schematically an illustrative exampleof a closed cycle MHD generator in accordance with the principle of thisinvention. In this system, monatomic (noble) gas at relatively low gaspressure, (i.e., 10 to 30 pounds per square inch absolute) for the sakeof tolerable recombination/attachment loss, is supplied to the MHDgenerator proper 50 after being heated by a suitable primary heat source52 such as a gas cooled nuclear reactor, conventional fossil fuelburner, solar heat source, or other means. The hot gas at the exhaust ofthe primary heat source or inlet to the MHD generator is at maximumsystem temperature, T typically, in this invention, at about 2,500 F.This gas temperature does not tax the endurance of the associatedmaterials exposed to the hot gas, yet it provides the desired cycleefficiency, typically about 45%. However, this gas temperature does notimpart significant ionization to the gas alone to make it an adequateelectrical conductor as required, namely about 10 or more mhos/meter.

The hot gas can be directed at subsonic gas velocity, for the sake oftolerable frictional pressure drop, to (a properly convergent-divergent)MHD generator duct, shown schematically at 50, for simplicity as astraight duct. Here it is electrically uniformally ionized bysimultaneous crossed field electron beam injection and periodic ionizingelectric field pulses below the sparking limit as previously described.This gas ionization is at the expense of a tolerably small fraction ofthe MHD generator output.

The MHD generator 50 directly converts physical or axially directedkinetic energy of charged particles and neutral particles (viacollisions with positive ions) into electrical energy, by virtue ofdeceleration of the charged particles (i.e., transfer of their kineticenergy into (induced) electrical energy in the coupled circuit). Inparticular axial rather than transverse voltage and current or powerextraction or coupling is utilized in this system, it being understood,however, that this invention is not limited to this type of powerextraction or coupling. In effect, the Faraday generator principle andOhms law apply, providing one takes cognizance of ion slip, Hall effectand the fact that the ions are not longer anchored as in a crystallattice of a solid electrical conductor.

The gas exhaust of the MHD generator is directed into a heat exchangeror re-generator, 54, wherein the residual heat of the gas is thermallyconducted to a preheater, 56, which heats the gas that is being returnedto the primary heat source, 52, to repeat the cycle. This measureimproves the cycle efficiency or otherwise conserves heat energy. Theresidual heat of the cooled gas from the re-generator is directed to acooler or refrigerator 58, wherein the gas is further cooled to thelowest practical temperature, T in order to obtain the maximum cycleefiiciency, regarding the system as a carnot heat engine. From thecooler, 58, the gas is passed to a primary gas purifier, 60, wherein theresidual gas impurities, other than the chosen gas, may be remove-d. Thecooled and purified gas is passed to the properly convergent inlet of anMHD motor or pump or accelerator or compressor, 62.

This device can be similar to the MHD generator 50, except that it isoperated as a motor by application of appropriate potentials to itsterminals. The gaseous electrical conductor is then electrically forcedalong its (properly divergent) duct, obtaining supplementary electricalenergy needed to return the gas to the primary heat source from the MHDgenerator, 50, by connections not shown in this figure, but detailed inFIG. 25 as an example. The discharge from the motor or pump oraccelerator or compressor, 62, is directed to the preheater, 56, whereit is preheated by the exhaust heat from MHD generator, 50, asaforesaid, before it is returned to the inlet of the primary heatsource, 52, where the cycle is repeated.

In some cases, it may be desirable to substitute another type of pump oraccelerator or compressor the MHD type. However, when freedom frommoving mechanical parts, absence of lubrication problems, etc., areimportant considerations the MHD device has decided advantages.Electrical input to the motor, 62, can if desired, be supplied from anysuitable electrical source other than generator, 50.

The gas purifier, 60, can be of any suitable type, electrical, physicalor chemical or a combination thereof.

FIG. 3, illustrates elementary relations between various componentsinvolved in the MHD generator and MHD motor action. In the figure, vrepresents vectorially the gas velocity and B the applied magneticfield, the latter directed perpendicular and out from the plane of thepaper. Due to the forcing of charges through the magnetic field by thegas stream via axially directed elastic collisions with them and theelectrostatic forces between the electrons and positive ions, anelectric field is induced at right angles to both the velocity and themagnetic field vectors shown vectorially as E, equal to E XF. Theresulting induced current density is shown as IT, and is equal to @E,where a is assumed (approximately) as a scaler (rather than properly asa tensor) electrical gas conductivity, assuming an isotropic plasma inspite of the magnetic field, for simplicity of illustration. Due to theaction of the plasma being forced through the magnetic field an inducedEMF 1 results equal to 7 x1? directed parallel but opposite to v asshown.

The figure also shows that in the case of a motor, an applied electricfield intensity E directed oppositely to T, but orthogonally to E and 5The resulting conduction current I] due to the applied E is equal to 0E.The net measurable current density through the plasma is consequently je-(F-FE XF). The action of the current density I gives rise to a Lorentzforce F, of value 7X? or (E-kflxl?) X15. In case IE is greater inmagnitude than E, the device acts as a motor; otherwise it acts as agenerator.

The flow of electrons, e, and positive ions, i, in the interactionspace, together with the resulting ionization of the gas is alsoillustrated in FIG. 3.

For continuous Faraday electrodes and Hall effect a phase angle ofcurrents with respect to the electric fields occurs. The magnitude ofthis phase angle depends upon the ratio of the electron cyclotronfrequency to the collision frequency, whereas the electric fields remainas indicated. For segmented Faraday electrodes and negligible Halleffect, the currents are as shown. With Hall effect, a phase angledeparture of electric fields with respect to the currents occurs asshown in the vector diagrams of FIGS. 1A, 1B, 1C.

Longitudinally coupled generator FIG. 4 shows schematically anelementary embodiment of the MHD generator 50 of FIG. 2. The principleof operation depends upon the Hall effect which is predominant andrequires electric coupling of, or power extraction by, longitudinal oraxial voltages and currents, as in the Hall-type configuration shown inFIG. 10, while the transverse currents provide the necessary physicalresistance to the ionized gas flow. Neutral gas of high electronmobility characteristics, namely a monatomic or noble gas such as heliumor argon at lower than atmospheric pressure, so thatattachment/recombination losses are not excessive, enters the generatorfrom the primary heat source at the left, as shown in the figure by anarrow 64. In an illustrative example, the compromising temperature,taking cognizance of material limitations and good cycle efliciency, ofthe gas is chosen about 1645 K. (2500 F.) and for the sake of tolerablefractional pressure drop, its speed subsonic, at about 1000 meters/second.

The gas first passes an unobstructing primary crossed field electron gunregion 66, between a cathode assembly, comprising an electron emitter68, a Wehnelt electrode 102 and control grid 63 and an acceleratorelectrode of the electron gun which is shown as a short type crossedfield electron gun. The primary crossed field electron gun controls thegas ionization by means of controlled electron beam injection andperiodic ionizing electric field pulses as previously described so as tominimize excess ionization within it by application of proper ionizingpulse duration and repetition rate and electric gradient at the cathodeaperture, to minimize recombination energy loss.

With reference to FIGS. 4 and 4a through 4e, that the anodes (shownshorted to the emitting soles, for longitudinal coupling) actuallyperform dual functions. That is, the anode accommodates not only theoutput power (terminals) and shorted transverse voltage and current(shorting straps), but also independently the pulsed ionizing voltage,with respect to the emitting soles; identically with that appliedbetween the accelerator and cathode of the primary crossed fieldelectron gun.

To avoid interference between these two functions, the anodes arestructurally composite and independent or,

alternately, blocking or chokes are inserted in the output terminals andshorting straps so as to not only prevent the flow of pulse currentthrough these circuits, but to prevent shunting of the pulse voltagethat is applied between each anode and sole pairs.

The composite anode may comprise one set of interdisposed and connectedelements for the pulse voltage, and the other set for the output powerand shorted transverse voltage and currents. In the alternate meansinvolving blocking or chokes, these can be inserted in the outputterminals and shorting straps so that the subject pulsing voltageapplied to the anodes behaves as though the output terminal and shortingstraps are absent, yet does not interfere with their individualfunctions.

Thereby, use of appropriate pulse voltage duration (to prevent excessgas ionization and hence sparking and arcing), appropriate pulse voltageamplitude (to raise the electron temperature in appropriate increments),and appropriate pulse voltage repetition rate (to compensate for theionization decay rate), provides encouragement for achieving a uniform,continuous and adequate gas ionization throughout the effectiveinteraction space.

The subject pulsed electric field gas ionization means supplements thatachieved by means of the electron beam. Thus, the two non-equilibriumgas ionization means working together provide promise of attaining thedesired range of electrical gas conductivity and other benefitspreviously mentioned.

Furthermore, working at high Hall parameter enables a corresponding highoutput (power) impedance so that the cathode current density is verylow.

Furthermore, use of secondary electron emitting soles with highsecondary electron yield eliminates electron emission problems. Theseemitters take advantage of the back bombardment characteristic ofcrossed field operation of emitters.

Various type crossed field electron guns, as illustrated by FIGS. 6 andl720 in accordance with the principles of this invention, can besubstituted for the type of gun shown. It should be noted that althoughFIGS. 6 and 17 illustrate primary cathode heaters, this is not necessarysince the hot gas is at adequate temperature for the cathodescontemplated, especially since the nonthermal electron emissionproperties of the cathode are emphasized.

The electron gun in FIG. 4 employs crossed periodic pulsed ionizingelectric field and direct current magnetic fields, the periodic pulsedionizing electric field being provided by means of a source (i.e., MHDgenerator) of electric potential represented arbitrarily by a keyedbattery, 72, connected between the accelerator, 70, and the cathodeassembly 68, 63, 102. This cathode assembly with or without the controlgrid, 63, and Wehnelt electrode, Hi2, may comprise (not shown) theemitting soles, 78, 84, 91 if desired.

The magnetic field can be provided by one or more conventional orsuperconducting solenoids (or by one or more permanent magnets, notshown, which are not likely to be practical due to the high MMFrequirements of Hall generators). The magnetic field distribution isgenerally substantially uniform throughout the MHD interaction space andthe electron gun regions of the MHD generator (and MHD motor). However,certain situations may indicate modified (non-uniform) magnetic fielddistribution at the ends of the electrodes along the flow so as toincrease the local charge density. The magnetic field is indicated by anarrow, 74, shown perpendicular to the paper.

The applied electric field in the electron gun region is governed byPaschens and crossed field laws. The effects of the former law aremitigated (by utilizing pulse durations of the order of a nanosecond attypical nonequilibrium MHD conditions) and controlled so as to increasethe gas conductivity beyond the limit of electron beam injection aloneby utilization of periodic pulsed ionizing electric field at appropriatepulse duration (e.g., order of decimicrosecond as previously discussed),and at a repetition rate to mitigate ionization decay rate and for thesake of full (100%) duty operation, and to avoid the deleterious eifectsof gas discharge and consequent non-uniform gas ionization. Furthermore,the relative intensities of the electric and magnetic fields can be suchthat only insignificant electron current flows to the accelerator, 70,and consequently quite modest electron current need by demand of theillustrative periodic pulser, 72, in energizing the electron gun.Provision against reverse electron current can also be made by meanscommon to the state of the gas discharge art.

A control grid electrode, 63, is incorporated for alternating currentoperation. It is used with a Wehnelt electrode, 102, as shown inconjunction with the cathode, 68, the electrode, 63, being appropriatelybiased, (i.e., with intermittent (pulsed) or alternating potential) withrespect to the cathode, by means of a biasing source representedarbitrarily in the figure by modulator 65. The purpose of the electrode,63, is to control the electron beam. Although separate sources for theelectron gun are illustrated, these very low power sources of potentialare energized by the MHD generator. The cathode, 68, is a non-thermaland thermal type electron emitter, in the latter case deriving its heatfrom the hot gas flow in which it is in contact.

Electrons emitted from the cathode, 68, are drawn away toward theaccelerating electrode, 70, under the influence of the periodic pulsedelectric field and direct current magnetic field, by application ofappropriate sources mentioned-In addition, modulation of the grid foralternating current operation can be included. Thereupon the electrons,except for any captured by the accelerator electrode, are immediatelyconstrained to move in magnetron paths, due to the combined eflfect ofthe orthogonal crossed electric and magnetic fields.

The pulsed electric and continuous magnetic field intensities arepreferably adjusted so that the paths of the electrons in the gasstarting at the extreme left of the cathode will just barely graze theaccelerator plate and will not be substantially collected thereby butwill be further constrained to continue substantially parallel to theaccelerator plate in accordance with crossed field constraints, before,at, and after passing the throat of the electron gun. The result issubstantially a smooth electron flow forming a stream of electronsemerging from the throat of the electron gun, in magnetron paths, allsubstantially along the plane of the electrodes.

FIGURE 4 shows schematically the electron stream from the primarycrossed field electron gun, 66, as it is injected into the gaseousstream in the interaction space of the generator. In the interactionspace, the moving gas passes between a series of electrode pairs anode,76, cathode, 78, anode, S2, cathode, 84, anode, 88, cathode, 90, etc. ofparallel electrodes separated by insulating spaces without plates. Theupper plate in each pair, as shown in the figure serves as an anodewhile the lower plate of each pair serves as a crossed field cathodesimilar to that of the electron gun and is referred to as electronemitting sole. The emitting soles are non-thermal and thermal orthermionic type (i.e., cermet or dispenser) type-deriving their optimumoperating temperature for the thermal or thermionic portion for electronemission from the flowing hot gas with which they are in contact. Theycan also be, as the cathode of the primary electron gun, indirectlyheated to meet any additional heat requirement on account of anyinadequacy on the part of the hot gas. The emitting soles are preferablyhighly productive of secondary as well as primary electrons.

A succession of many short electrodes is employed instead of longcontinuous electrodes in order to break up the longitudinal circuit andso prevent large axial or longitudinal surface currents which wouldotherwise flow in the electrodes, due to the Hall effect which is madedominant for the purposes of this invention, a circumstance which wouldcause large ohmic losses. Furthermore, since at high magnetic fields orhigh Hall effect parameter, the current density tends to peak at thetrailing edges of the electrodes, appropriate design considera tionshave been applied to minimize erosion thereat.

The emitting soles 78, 84, 90, etc. supply the electrons to the gas orplasma in the interaction space as required to make up for electron lossand those passing to the :load. To short circuit the electrodes of eachelectrode pair, electrical conducting straps, 80, 86, 92, etc., areprovided outside the interaction space between the anode and theemitting sole. The axial or longitudinal current, voltage and power aretaken otf through a load circuit, 94, connected between the first andlast pairs of electrode pairs, as shown. The magnetic field, both in theelectron gun and in the interaction space is in the same direction butnot necessarily the same magnitude and perpendicular to the plane of thepaper and that of the ionized gas fiow and electron beam flow. Both theapplied magnetic field and the injected electron beam are made assubstantially uniform as possible throughout the interaction regionsbetween facing pairs of electrodes. However, there may be an advantagein controlling the magnetic field distribution in the interaction spacewith respect to the vicinity of the edges of the electrodes to takeadvantage of the increase in local density of charges thereat and soincrease the power output from the MHD generator.

The use of the electrical ionization via electron beam injection,supplemented by periodic pulsed ionizing electric field means aspreviously discussed for maintaining adequate and uniformly as possibleionization of the flowing gas so that the desired electrical gasconductivity is achieved in the interaction spaces of the MHD generator(and the MHD motor), renders the system as a whole self-exciting andself-starting, as in the case of an electromagnetic machine orself-excited thermionic oscilator. With the gaseous stream forcedthrough the duct, ionization is established immediately upon enteringthe primary crossed field electron gun portion of the duct. Full MHDgenerator (or MHD motor) action begins immediately upon theestablishment of adequate ionization in the flowing gas, which requirescontinual and uni form ionization throughout the volume of the duct.

Requirement for continual uniform ionization arises on account of therapid rate of decay of ionization, due primarily to recombination/attachment loss, which is characteristic of the behavior of the gas. Thedistribution and energy of the electron stream can be adjusted toaccomplish this desired result while at the same time its energy isvirtually entirely absorbed in the gas upon reaching the far end of theinteraction space and an electron collector and heat regenerator, 98,which is provided at that point.

The energy required to maintain the desired degree of gas conductivityby electrical ionization, namely conductivity in the order of mho/m. ormore, is moderate; it being of the order of a base percentage the usefulpower output of the generator. It is known in the art and as has beenpointed out previously that the electric power generated is less than apercent of the total energy associated with the gas. That is, the totalenergy is equivalent to a small fraction of an electron volt at typicalMHD conditions. Since the ionization potential of ordinary gases arerelatively high, only a few percent of the atoms at the most would beionized under ideal conditions of the available energy extracted andutilized. Nevertheless, the electrical conductivity of the gas whenionized to the extent of a small fraction of a percent is substantiallyas high as for the completely ionized gas.

A supplemental means for ionizing the gas with crossed field electronbeam periodic pulsed ionizing electric field means, at the expense of .asmall fraction of the power output of the MHD generator, is alsoutilized, as previ- 18 ously discussed to achieve the desired electricalgas conductivity.

Only a few of the required representative electrode pairs in thegenerating portion of the generator, 50, are shown for illustration inFIG. 4.

The pair of electrodes nearest the electron gun comprises the anode, 76,and the electron emitting sole electrode, 78, connected together outsidethe gas duct by the electrical conductive strap, 80, which is ofnegligible resistance. The opposing faces of the electrodes of each pairare parallel to the direction of the magnetic field. An intermediateelectrode pair comprises the anode, 82, and the electron emitting sole,84, connected by a like strap, 86. The pair of electrodes farthest fromthe electron gun comprises the anode, 88, and the electron emittingsole, 90, connected by a like strap, 92. The output terminals of thegenerator, 50, comprise the electrode pair, 76, 78, and strap, 80, whichconstitute the generator terminal conventionally called the negativeterminal, for direct current operation, and the electrode pair, 88, 90,and strap 92, which constitute the positive terminal. The electricalload connected to the generator terminals is represented in the figureby the impedance, 94.

Electrons emitted by the cathode, 68, follow, in the simplest case, haltcycloidal paths in the region between the cathode and the accelerator.The gun forms an electron beam which substantially permeatesuniforrnally the entire interaction region of the gas duct as previouslydiscussed. The electron beam moves to the right as shown in the figure,while ionizing the flowing gas, moving in the same direction, with theaid of the applied periodic pulsed ionizing electric field betweencathode and anode or accelerator. In the interaction portion of thegenerator, the plasma is subjected to a strong Hall eifeet, whichinduces both transverse and axial or longitudinal current and voltage inthe plasma. The transverse currents serve to provide the necessaryphysical resistance to flow. The load, 94, is coupled through thelongitudinal currents and voltages. Thereby, desired high rather thanlow impedance output characteristics are realized.

The electron beam injection and applied pulse duration, amplitude andrepetition rate of the ionizing electric field can be controlled so thatthe ionization effect of the electron beam and pulsed electric field issubstantially uniform and limited to the desired interaction region ofthe gas duct. After traversing the interaction region the plasmadeionizes very rapidly as previously discussed, due primarily torecombination/ attachment losses. Residual charges are collected at theion collection/regenerator 98, which in a very simple form can compriseone or more plates arranged edgewise to the gaseous stream. The ioncollector/regenerator, 98, primarily serves the purpose of absorbingresidual heat energy from the exhaust gas. The neutral gas leaving theion collector/regenerator, 98, is in turn exhausted toward the right inthe figure as indicated by the arrow, 100.

To adapt the MHD generator for alternating current component operation,an alternating voltage, represented by a modulator 69, is impressedbetween the control electrode, 63, and the cathode, 68, as shownschematically in FIG. 4. The action of the electron gun is the same asdescribed previously, except that the electron flow is modulated insteadof steady. For direct current operation the periodic pulsed non-ionizingelectric field bias voltage, 65, can be applied, but is optional. Itspulse duration is so short (order of a nanosecond) that no significantgas ionization occurs on its account. The alternating voltage frommodulator, 69, can be superimposed on this for alternating current poweroutput from the MHD generator. As a result, there is a controlledvariation of the degree of gas ionization and, hence, of the electricalgas conductivity in the MHD generator. This in turn causes acorresponding modulation of the power output of the MHD generator. Themodulation of the power output of the MHD generator is attributable tothe fact that the ions of the gas recombine extremely rapidly in theabsence of an effective process of continual ionization so thatsignificant variation of the electron output of the electron gun andperiodic pulsed ionizing electric field is reflected in correspondingvariations of the electrical gas conductivity and hence power output ofthe generator.

A switch, 67, is shown whereby the modulator, 69, can be included in thecircuit or not, depending upon whether direct current or alternatingcurrent component operation is desired. It will be evident that themodulated or alternating current output from the MHD generatorobtainable by use of the modulator can be converted by means of atransformer in conventional manner and that all known intermittent orpulsed or alternating current te hniques can be employed in theutilization of the power output of the generator. Direct currentoperation especially at commercial voltages, is applicable in practice.Alternating current operation could also be obtained, in principle atleast, alternatively by modulating the current in the solenoidsproducing the impressed magnetic field (especially in the case of asuperconducting solenoid) but with much more drive required (except inthe case of superconducting solenoids) and much less economy compared tolow drive voltage modulation of the control electrodes of the primarycross field electron gun and secondary crossed field emitting soles.

Double-walled duct embodiment FIGS. 6, 8 and 11 show an illustrativeembodiment of coaxial ducts, primary crossed field electron guns andelectrode structures for use in an MHD generator (or MHD motor)according to the invention. The emitting soles thereof can be replicasof the primary cathode assembly or can be simpler, as discussedpreviously. The coaxial duct is built on the Dewar principle with twospaced walls and thermal insulating space between the walls to minimizeheat loss from the inner duct. The inner duct wall is a suitabledielectric (e.g., ceramic) with matching metal (e.g., tantolium, etc.)inserts on the inside to form the electrodes and joining members ofsections. Those electrodes that are cathodes and emitting soles can bedirectly heated (via the hot flowing gas) or indirectly heated (via anauxiliary heater) thermionic and non-thermal electron emitter, aspreviously described. They can have a coating or a layer or comprise abody of electron emissive material such as a dispenser or cermet type.

Suitable dielectric (or ceramic) spacers are provided between the innerand outer walls at intervals along the duct for the purpose of isolatingand supporting the inner duct in the outer one. The outer duct wall isof suitable metal (e.g., stainless steel) with suitable dielectric (orceramic) inserts at locations where electrical connection is made to anelectrode inside the duct. The ducts can be made in demountable setcionswhich can be welded (e.g., heli-arc) together to make up any requiredlength, and separated (e.g., by grinding off the welds) a number oftimes and rescaled as before as desired.

FIG. shows in three-dimensional diagrammatical form a generalconvergent-divergent duct or nozzle which involves consideration of theJoule-Thomson (Ioule-KeL vin) effect. Such nozzle can be used inpractice for converting the random motions of the gas into more axiallydirected motion within the electron gun and interaction spaces in thegas duct in an MHD converter. In the near vertical wall of the divergentportion of the duct are shown schematically the crossed field electrongun cathode 68 the first sole electrode 78 in the interaction space, forsimplicity a few intermediate sole electrodes, and the last electronemitting sole electrode, 90. Usually in practice, a much larger numberof intermediate emitting sole and anode electrodes will be required,depending upon design requirements. In the far vertical wall of thedivergent portion of the duct are shown schematically the electron gunaccelerator electrode 70, the first anode electrode 76 in theinteraction space, a few intermediate anode electrodes, and the lastanode electrode, 88. The sole electrode of each opposing pair ofelectrodes in the interaction space in shown strapped to the respectiveanode electrode of the pair, the first strap and the last strap 92constitute respectively, the negative terminal and the positive terminalfor the load impedance 94 for direct current operation. The relativedirections of the gas velocity, v and the magnetic field intensity, B,are shown by arrows in the figure.

FIG. 6 shows a longitudinal or axial cross section of a simple MHDgenerator. The coaxial ducts therefore, in a central plane perpendicularto the magnetic field. This plane is also perpendicular to the planes ofthe opposed electrodes. FIGS. 7 and 8 show external appearance at sideand one end, respectively. Although a conventional straight duct systemis illustrated, in practice, the inner duct may be tapered, for exampleas illustrated in FIG. 5, so that where C and /gf denote cross sectiondimension and final gas velocity, respectively. The dielectric (orceramic) inner duct wall that is uppermost in the figure is designated,200, and the lower inner duct wall is designated, 202. The upper metalduct outer wall is designated, 204, and the lower, 206.

The electron cathode assembly of a simpler illustrative type of crossedfield short electron gun suitable for direct current operation is showngenerally at 208. The cathode of the gun need not have an indirectheater assembly since the hot flowing gas is at adequate gas temperaturefor optimum thermionic operation. Other electron guns (e.g., thosedisclosed in FIGS. 17 through 22) for alternating current operation canbe substituted for the electron gun shown. A simplified cathode 210 isshown comprising electron emissive material either coated upon asuitable base metal or comprising a cermet or dispenser type unit. Thecathode 210 can operate by virtue of heat of the hot (e.g., 2500 F.)inert gas or its own accord, thermionically by means of a suitableintegral electrical heat source. It can also be primarily a non-thermaltype of emission cathode.

The indirectly heated (optional) cathode 210 is shown in the form of aninverted metal box separated on all sides from the main portion of theinner wall 202 by a gap 212. Inside the cathode, 210, are located aplurality of electrically heated heater elements indicated schematicallyat 214. All the heater elements 214 are connected in parallel inconventional manner and brought out through an insulating tube 220 to apair of heater connections 222. The heaters are held in place by a coverplate assembly 224 through which the tube, 220, projects.

A Wehnelt electrode, 226, generally surrounds the sides of the cathode210. The electrode 226 comprises a sheet of metal inserted into the wall202 to the left of the cathode, folded downward over the edge of thewall 202 at the gap, 212, passing under the cathode and back up oppositethe cathode at the right of the cathode to fasten to the wall 202 at theright of the gap 212. A suitable dielectric (or ceramic) spacer ring 228permits the cathode to be insulated and supported by the controlelectrode. An access side duct 230 for cathode connections is insertedin the lower outer wall 206. The duct 230 is provided with a suitablecover plate 232 and a suitable dielectric (or ceramic) electrode basepress 234, sealing a hole in the plate 232, through which the heaterconnections 222, a cathode connection 236 and a control electrodeconnection 238 are brought out. The modulator 69 and the biasing voltage65 can be inserted when required between the cathode connection 236 andthe control electrode connection 238 in a manner similar to that shownin FIG. 4 between the electrodes 68 and 63 of that figure.

The accelerator for the elementary short electron gun is shown generallyat 240. The accelerator plate 242 is a metal (e.g., tantalum) plateinserted (i.e., brazed) in the upper inner duct wall 200. The plate 242extends opposite the control electrode 226 and the cathode 210.Electrical connection is made to the plate 242 by way of a metallic pin244, which can be integral with the plate, the upper end of which makesa suitable engagement with a nut 246. An access hole in the upper outerwall 204 opposite the nut 246 is sealed by a metal-dielectric (e.g.,metal-ceramic) plug assembly 248, which can be sealed (e.g., heli-arcwelded) to a metallic collar 250. Through the plug assembly 248 extendsa shouldered metallic pin 252, the lower end of which makes a (e.g., setscrew) locked engagement with a nut 254 to permit angular orientation ofthe nut 254.

Hairpin type electrically conducting wires 256 are inserted into holesin, or otherwise secured to the peripheries of the nuts 246 and 254.Aset screw (not shown) is provided in the nut 254 for securing the nut incorrect angular position on the pin 252 so as to align the placements orholes for the wires 256. A plan view of the nut 246, hairpin wires 256and without the set screw is shown in FIG. 9. The shouldered end of thepin 252 is provided with suitable means for engaging an externalelectrical connector 260.

The metal and ceramic parts are joined in vacuum-tight manner as knownin the art, so that the space between the inner duct and the othercasing can be evacuated by known means and operated under high vacuum toreduce heat transmission from the inner duct to the outer casing and theambient medium.

A sole electrode for the electron gun is shown at 262. It can be anelectron emitter or not, depending upon the application. In any event,its construction can be simple for direct thermionic electron emissionby means of the hot gas or it can be primarily a non-thermal electronemitter independent of the hot gas. Its form can be generally similar tothat shown for the accelerator electrode 240.

An auxiliary anode electrode for the electron gun is shown generally at264. The construction of the auxiliary anode electrode can besubstantially the same as for the accelerator 240. The auxiliary anodeelectrode is flush with the upper wall of the duct as shown.

An anode electrode, for either an MHD generator or an MHD motor with anelectrical connection external to the duct, is shown generally at 266.The construction of this anode electrode can be substantially the sameas shown for the electrode 240, except for current capacity and becauseat the high Hall parameter or magnetic field characteristic of the Hallgenerator (FIG. 1C) the trailing edge of the anodes (and cathodes) withrespect to the gas flow must be made more erosion resistant due to theconcentration of electric current there. The anode electrodes areinserted into the upper wall of the duct. In an MHD generator, theexternal electrical termination of anode electrode 266 is used only atthe two generator terminals that are to be connected to the electricalload of the generator.

The first of these, from left to right, is the negative terminal of thegenerator for direct current operation. It is the first anode electrodeto the right of the primary electron gun. The second terminal is thepositive terminal of the generator and is the last anode electrode showngenerally at 268 at the extreme right hand end of the interaction spaceof the generator, the construction of which is substantially the same asshown for the other anode electrodes that have external electricalconnections. In an MHD motor, all the anode electrodes and emittingsoles may require external electrical connections.

The electron emitters of the cathode of the electron gun and sole in theMHD generator can be operated either as a non-thermal thermionicemitters, actuated by the passing hot gas stream, or as indirectlyheated emitters, actuated by impressed external heat source, shown inthe drawings only for the cathode of the primary electron gun.

The magnetic field can be readily supplied by permanent magnets,however, because of the high mmf. requirements it may be desirable togenerate the magnetic field by means of electromagnets, conventional orsuperconducting.

The non-therrnal thermionic electron emitters can be refractorydispenser or cermet types, having an operating temperature of about theambient gas temperature (or 2500 F.). Operating these emitters at theiroptimum temperature insures long operating life.

An anode electrode that has no electrical connection through the outerduct wall is shown generally at 278. The structure comprises a metallicplate 272 inserted in the upper inner duct wall 200. A metallic pin 274which can be integral with the plate 272 makes electrical contact withthe plate 272 on the upper face thereof and extends upwardly to theupper surface of the wall 200 into a strap groove 302 (FIG. 12) forconnection to a strap 288, as will be explained presently with referenceto FIG. 11.

An electron emitting sole electrode that has no electrical connectionthrough the outer wall of the duct is shown generally at 276 (FIG. 6).The structure comprises a composite metallic plate 278 inserted in theinner lower wall 202 of the duct. The upper surface of the compositeplate 278 can be impregnated or coated with a thermionic or secondaryelectron emissive material to promote copious emission of electron asrequired. The strap 288 lying in a strap groove 302 (FIG. 12) isconnected to the plates 278 and 272 as shown in FIGS. 11 and 6.

An anode electrode having no electrical connection through the outerwall of the duct and having a thermal and electrical isolator and spacerspanning the region between the inner and outer walls of the duct, isshown as an example at 286. The structure is generally similar to thatshown for the anode electrode 270, except that a pin corresponding tothe pin 274 extends sufficiently beyond the upper surface of the wall208 to form a support or axle for a thermal spacer such as a wheel 282.The rim of the wheel 282 is displaced laterally from the axle to such anextent that the rim bears against the inside surface of the upper outerwall 204, while the axle bears against the outer surface of the upperinner wall 268. The spacer or wheel 282 can have a plurality of spokeswhich extend diagonally between the axle and the rim.

An electron emitting sole electrode, similar in size to the anodeelectrode 280, is shown generally at 284. This electrode is insertedinto the lower inner wall 202 of the duct and the thermal spacer orwheel 286 corresponding to the spacer or wheel 282 spans the regionbetween the outer surface of the inner lower wall 202 and the innersurface of the outer lower wall 2%.

FIG. 11 shows a cross section of the duct through the anode electrodestructure 286 and the electron emitting sole electrode structure 234.The view ShOWs the inner and outer walls of the duct in cross-section,together with the thermal and electrical isolators and spacers shown asthe wheels 282, 286. The view further shows the short-circuiting strap288, which corresponds generally to any one of the straps 80, 86, 92shown schematically in FIG. 4 for connecting an anode electrode to anopposing electron emitting sole electrode by an electrical conductivepath outside the inner wall of the duct; specifically over the outersurface of the inner wall of the duct. The anode end of the strap 288 iselectrically connected to a metallic pin 290 that forms the support forthe isolator and spacer wheeel 282. The strap 288 lies in a groove inthe outer surface of the inner wall 260' of the duct, extending upwardlyas shown in FIG. 11 and over the top outer surface of the inner wall,and thence downwardly to an electrical connection with the metallic pin292 that forms the support for the isolator and spacer wheel 286.

1. IN A PLASMA ENERGY CONVERTER, APPARATUS FOR GENERATING IONS COMPRISING MEANS FOR GUIDING PLASMA ALONG A PREDETERMINED PATH, SAID CONDUIT MEANS INCLUDING A GAS INPUT MEANS AND A GAS OUTPUT MEANS, A SOURCE OF HEATED NEUTRAL GAS, MEANS FOR CONNECTING SAID SOURCE OF HEATED GAS TO SAID GAS INPUT MEANS SO THAT THE HEATED GAS FLOWS THROUGH SAID CONDUIT MEANS, MEANS FOR INJECTING AN ELECTRON BEAM INTO A GIVEN REGION WITHIN SAID CONDUIT MEANS, MEANS FOR APPLYING A MAGNETIC FIELD TO AT LEAST SAID REGION WITHIN SAID CONDUIT MEANS, SAID MAGNETIC FIELD HAVING A DIRECTION SUBSTANTIALLY PERPENDICULAR TO SAID PREDETERMINED PATH, MEANS FOR APPLYING A PULSED ELECTRIC FIELD TO SAID REGION, SAID PULSED ELECTRIC FIELD HAVING A DIRECTION SUBSTANTIALLY PERPENDICULAR TO BOTH THE DIRECTION OF SAID MAGNETIC FIELD AND SAID PREDETERMINED PATH, SAID PULSED ELECTRIC FIELD HAVING A PULSE DURATION OF LESS THAN TEN NANOSECONDS TO FIELD IONIZE SAID NEUTRAL GAS WITHOUT BREAKDOWN WHEREBY THE NEUTRAL GAS IS IONIZED PARTLY BY THE INJECTED ELECTRON BEAM AND PARTLY BY THE FIELD IONIZATION RESULTING FROM THE PULSED ELECTRIC FIELD. 